This invention relates generally to techniques for scrambling and unscrambling a video signal and, more particularly, to techniques for compensating for distortions in a video signal caused by the scrambling and unscrambling of the video signal.
Secure transmission of video signals is becoming increasingly important with the growing use of video teleconferencing, cable TV and satellite TV transmissions, and with the advent of direct-broad-cast-satellite (DBS) transmission. Different techniques have been developed for providing various levels of video security, with corresponding levels of complexity and cost. One technique that provides a relatively secure video signal with a modest amount of complexity and cost is line spin, or line rotation, scrambling. Line spin scrambling is performed in an encoder by making a cut in the active portion of a video line at a breakpoint determined by a pseudorandom number generator. The two segments that result are then interchanged, while the horizontal and vertical synchronization and blanking intervals are left intact. After transmission and reception of the video signal, the signal is unscrambled in a decoder by reversing the line spin scrambling that was performed in the encoder. The breakpoint is determined in the decoder by a pseudorandom number generator that is identical and synchronized to the pseudorandom number generator in the encoder.
Unfortunately, line spin scrambling suffers from certain distortions that corrupt the unscrambled video signal. One of these distortions is caused by a sawtooth-shaped voltage error introduced into each line of video during demodulation of the video signal. The phase of the sawtooth waveform is such that a linear charge ramp occurs during the horizontal blanking interval, and a linear discharge ramp occurs during the active portion of the video line. The linear discharge ramp, or line tilt, will corrupt the active portion of any demodulated video signal, but the effect on a received picture is generally acceptable when the video signal has not been scrambled. This is because the amplitude and phase of the line tilt are approximately the same for all horizontal lines and, therefore, the effect across the received picture is constant in the vertical direction and is a gradual luminance variation in the horizontal direction. However, a video line that has been scrambled with the line spin technique has the full amplitude of the line tilt applied at a single point, where the two segments are pieced back together during line spin unscrambling. This causes a sharp luminance discontinuity that occurs at the randomly chosen breakpoint in each video line. The result is a chaotic hashing of luminance striations in the received picture. Accordingly, there has been a need for a technique that compensates for this type of distortion in the video signal. The present invention fulfills this need.